Diarrhoea is a frequent problem in the equine population, with specific aetiologies infrequently diagnosed. Therefore, treatment will often be empiric, but specific treatment modalities can be instigated if the aetiology is known or suspected. The approach to treatment for these cases will be discussed further and broken down into non-specific treatments and specific treatments.
Clostridium difficile and perfringens
Antibiotic therapy should be instigated when there is clinical concern over an infectious bacterial aetiology but also weighed up against the increased risk of further dysbiosis. In most clinical cases of diarrhoea (unless severely septic), the author avoids the use of broad spectrum or penicillin-based antibiotics. Cephalosporins should be avoided in all cases due to their increased risk of inducing a clostridial diarrhoea alongside their protected status (O’Connor et al., 2004).
When considering clostridial disease, the mainstay of antibiotic therapy will be metronidazole, with doses ranging from 15 to 30mg/kg q12h PO (the author generally uses 25mg/kg q12h PO). Metronidazole has also been shown to have an anti-inflammatory effect on the gastrointestinal tract as a beneficial side effect and therefore can play a role in inflammatory bowel disease cases.
Some horses will show anorexia secondary to metronidazole; administration per rectum (increase the dose by 25 percent) can ameliorate these signs (Steinmann et al., 2000).
Di-tri-octahedral smectite has been shown to be effective at absorbing and neutralising endotoxins, particularly those from clostridia, and therefore should be given in any case where there is a potential for either Salmonella or clostridial disease. The first dose should be 3g/kg followed by 1g/ kg q6h PO, although many horses won’t tolerate this dose without stomach tubing. If performing nasogastric intubation, the veterinary surgeon should be careful to not have any oral contact with the nasogastric tube due to the risk of zoonotic infection.
If Salmonella is considered, or proven to be, a pathogen in a clinical case then gentamicin is a suitable monotherapy. Its penetration into the GI tract is limited but will reduce the rate of translocation and therefore the risk of sepsis and SIRS. The dose should be 6.6mg/kg IV q24h but if there is severe oedema secondary to hypoalbuminaemia, the dose may need to be increased. Monitoring of the therapeutic levels (both a trough and peak) can ensure adequate dosing as well as reducing the potential nephrotoxic nature of the medication.
Encysted cyathostomins and parasitism
Parasitism is a very common cause of diarrhoea within the equine population, particularly in the UK. The mass emergence of encysted cyathostomin larvae can lead to a severe, life threatening diarrhoea requiring anthelmintic treatment. It should be noted that the use of these products can initially make the clinical signs, including hypoalbuminaemia, far more severe but must be undertaken to allow for resolution of the disease.
Parasitism is a very common cause of diarrhoea within the equine population, particularly in the UK
Fenbendazole (7.5 to 10mg/kg q24h for 5 days) has historically been used as the treatment protocol but given the very high rate of resistance seen throughout the UK and the world, its use should be limited. Instead, a tendency to use either ivermectin or moxidectin-based products should be undertaken and with the increased duration of action of moxidectin, this should have an increased kill rate. The encysted larvae, though, are metabolically inactive and as such are relatively resistant to any anthelmintic; repeat dosing is required to ensure most parasites are killed.
If a diagnosis of encysted cyathostomins has been made, the horse is clinically sick and there is a hypoalbuminaemia then glucocorticoid pretreatment should be considered. The theory is to reduce the amount of inflammation within the GI tract secondary to killing the encysted cyathostomins and reduce the risk of hypoalbuminaemia. Prednisolone can be used at 1mg/kg q24h PO or dexamethasone at 0.1mg/kg q24h IV.
During endotoxic episodes, there is a rationale in using flunixin at 0.25mg/kg q8h IV to ameliorate the cardiovascular effects of endotoxaemia. When starting NSAID therapy, consideration should always be given to the side effects on the colon and the kidneys and an informed clinical decision made.
When used at sub-therapeutic antibiotic doses, polymyxin B has been shown to have excellent endotoxin binding effects and therefore, in those cases that present with sepsis, it is well worth considering administering polymyxin B (1,000 to 6,000 IU/kg q8h IV). If there is concern about renal perfusion or damage, then the use of polymyxin B should be considered carefully due to its marked nephrotoxicity.
More frequently used as a prokinetic in ileus cases, lidocaine can be used both for pain relief and anti-inflammatory/anti-endotoxic cases. Its use is obviously isolated to those cases in hospitals where drip pumps can be used to reduce the risk of toxicity.
Inflammatory bowel disease
Following diagnosis based on biopsy or ultrasonography, glucocorticoid therapy will be the lynchpin of treatment. It should be noted that the oral bioavailability may be reduced in IBD cases and that treatment with intramuscular or intravenous administration of medications might be warranted.
Prednisolone (1mg/kg q24 PO) is an appropriate starting regime and if an excellent response is seen then a gradual tapering of the medications can be undertaken. The author normally undertakes a month of full dose treatment followed by tapering over another one to two months depending on the outcome. If clinical signs do not improve then it is worth considering treatment with dexamethasone (0.1mg/kg q24h IM or IV) to bypass the reduced bioavailability.
When monotherapy with glucocorticoids has failed then azathioprine can be added to the protocol, initially at 1mg/kg q24h and increased up to 3mg/kg q24h if required. Repeat monitoring of the white blood cell count should be performed as azathioprine can lead to a lymphopaenia. The author frequently uses azathioprine and glucocorticoids together until clinical signs improve and then gradually tapers steroids then azathioprine.
Increase gut transit time
Once an infectious aetiology has been ruled out as the cause of diarrhoea, pharmaceuticals can be used to increase gut transit time and allow for an increased rate of fluid absorption. Codeine phosphate can show a good clinical response, although the response is variable depending on the horse. Initial dosing should be 1mg/kg q12h and this can be increased up to 3mg/kg q8h depending on the clinical response.
Monitoring the rate of faecal production is essential as there is a risk of impaction following administration. If clinical resolution is seen then the dose can be gradually weaned off, or if there are concerns over an impaction, treatment can be stopped immediately. Ensure that the owner keeps a diary of faecal output to be able to assess the response objectively rather than subjectively.
Fluid therapy is vital to diarrhoea cases with a large volume of water being passed in each diarrhoea episode. It is not possible to fully assess the “ins and outs” in these cases, and instead various hydration parameters should be monitored (peripheral pulse quality, heart rate, capillary refill time, lactate, PCV and TP) and fluid therapy adjusted dependant on these. Hartmann’s will be the most appropriate therapy in most cases, but electrolytes and acid base abnormalities should be monitored with supplementation or fluid therapy changes instigated based on the abnormalities.
If hypoalbuminaemia is present, plasma transfusions will be required. Commercially made hyperimmune plasma is normally cost prohibitive and therefore plasma should be acquired from donor geldings. This is easily achieved with commercially available plasma collection kits. When collecting blood, a pre-collection PCV/TP should be performed to ensure that the donor is neither anaemic nor hypoalbuminaemic and then approximately 1.5 percent of body weight can be taken as whole blood (ie 7.5L/500kg). The catheter for collection should be placed “up the vein” to increase the flow rate of blood and the collected blood agitated with the anticoagulant during the whole procedure.
Multiple donors will often be required in severe cases due to a relatively small increase in plasma albumin achieved with a 4L transfusion of plasma. Reactions to plasma are infrequent but when starting each new donor’s plasma, the patient should be monitored for signs of a reaction, including tachycardia, tachypnoea, pyrexia and urticaria. If any of these are seen, the plasma rate should be reduced immediately, and administration of steroids considered.
There is a paucity of clinical research into the use of pre- or probiotics within equine patients. More recent research has looked at products that contain Saccharomyces with some evidence to show a reduction in the duration of the diarrhoea (Desrochers et al., 2005). Therefore, although not truly evidence based, the author will regularly put horses on a pre/probiotic during colitis. Other options can include placing faeces from a normal horse in the stable or even considering transfaunation of normal faeces.
The use of psyllium in sand enteropathies has some limited evidence of a beneficial effect at 0.5 to 1g/kg q12h PO (Hotwagner et al., 2008). It also has a beneficial production of volatile fatty acids that will help with mucosal health and therefore can play a role in any enteritis/colitis.
Misoprostol is a prostaglandin E1 analogue that is used in humans to induce labour and abortions as well as treat gastric ulceration. In equids, it has been shown to help ameliorate the side effects of NSAIDs in the colonic walls and possibly to help induce increased healing of the colonic wall which would, theoretically, be beneficial in any colitis case. The dose is 5mcg/kg q12h PO and in some rare cases, it can lead to a degree of colic and discomfort. Care should be taken when handling this product and appropriate warnings and labels applied to all products when given to owners.
No one treatment protocol is appropriate for all diarrhoea cases but with the above guidelines, a sensible course of action can be undertaken in most cases.